The present invention relates to a Fe-Ni alloy for shadow masks and a method for producing a shadow mask of such an alloy, and more particularly relates to production of a shadow mask made of a Fe-Ni alloy and used for colour television cathode-ray tubes or the like.
As a substitute for conventional low carbon rimmed steel and aluminum killed steel, invar alloys have recently been used as a material for shadow masks used for colour television cathode-ray tubes in order to meet large size and high grade demands for screen display. As well known, invar alloys are in general given in the form of Fe alloys containing about 36% by weight of Ni and exhibit relatively low thermal expansion.
Use of such invar alloys is proposed for the following reasons. In the case of a cathode-ray tube of a large size and high grade display, high electric voltage is applied to the cathode-ray tube to scan pores in the shadow mask and the amount of energy generated by electron beams impinging upon the shadow mask increases accordingly. When a shadow mask is made of the above-described conventional steels, heat generation caused by impingement of electron beams causes considerable thermal expansion of the shadow mask and such escalated thermal expansion leads to unfit landing of the electron beams on the shadow mask whilst resulting in undesirable colour slide in the screen display. Use of invar alloys removes such troubles thanks to their relatively low thermal expansion.
Despite such a merit, use of invar alloys is accompanied with other problems. Invar alloys are generally low in deformation resistance, i.e. low in Young's modulus. When used for a shadow mask for a television cathode-ray tube, the mask tends to perform resonance with sounds generated by the sound system of the television. In production also, low deformation resistance of invar alloys often induces easy buckling of the shadow mask during assemblage. This buckling problem is becoming very serious with recent trend of thinner shadow mask construction. That is, increasing high precision image display demand for cathode-ray tubes necessitates formation of many fine pores in the shadow masks by means of etching and the thickness of the shadow masks is made thinner and thinner for easy formation of such fine pores. Reduced thickness of the shadow mask allows easy buckling of them during assemblage. Further, prety high proof stress of invar alloys makes them unsuited for pressing process because considerable spring back occurring in pressing process results in defective shape of the products.